A doctor treating a severely disabled baby whose parents disagree over whether to remove his life support and let him die told the high court today that the boy's existence was "on a knife edge".

The paediatrician, who supervises the care of the one-year-old, known for legal reasons as Baby RB, said the boy was among the most severely disabled children he had looked after.

Dr F said that Baby RB's neuromuscular condition severely limited his ability to breath and move, which meant he was not equipped to survive without constant, intrusive and painful medical care.

"I find it very difficult and challenging caring for children on a daily basis who are as disabled as RB," he said. "To exist in this world you need some basic functions. You have to cough, to have gag reflex, to breathe and to respond to painful stimulation, and, in my view, RB has not adequately demonstrated any of these."

He told the court: "It troubles me that I am committing him on a daily basis to ongoing ventilation in an intensive care unit where I think that his daily existence is distressing and that he does not have the basic building blocks which I would see as necessary to live in the outside world."

RB, who was diagnosed with congenital myasthenic syndrome (CMS) after he was born last October, has never left hospital and relies on a ventilator to breathe.

His now separated parents disagree about what would be in their son's best interests. His mother supports the hospital's application to remove his life support on the grounds that his quality of life is almost non-existent. Lawyers for his father say Baby RB's brain is not affected by CMS and argue that he can see, hear, and recognise and interact with his parents.

Dr F said the boy's ability to survive was governed entirely by his life support apparatus. "His ventilator delivers him 20 or 22 breaths a minute," he said. "Every three seconds of his life he gets a flow of gas from the ventilator that inflates his lungs. That's very important. If he were only to get a breath every five seconds, he would immediately feel extremely breathless and distressed, and over a period of time, he would die. His life is on that much of a knife edge."

The father's lawyers have suggested that a tracheostomy operation to run a breathing tube directly into RB's throat rather than down his nose could help him and even allow him to live at home, albeit still on ventilation. But Dr F told the judge, Mr Justice McFarlane, that Baby RB was "not a candidate" for the procedure. He said the child would still be dependent on artificial ventilation, adding that if it became necessary to withdraw the breathing tube in an emergency, he would suffer far greater distress than if he were in hospital on a life support machine.

The doctor was visibly upset as he explained that efforts to try to save Baby RB in the former scenario could involve fractured ribs and injections directly into his bones. Dr F added that in other cases involving similarly disabled children, parents often decided that life support should be removed and palliative care provided until death.

"I think we owe it to these children as clinicians and parents to make sure they die a dignified death and that has to be planned," he said. Asked by Michael Mylonas, counsel for the hospital trust, if he thought Baby RB could let those around him know whether he was happy or in pain, Dr F replied: "His interaction is so marginal that it's very difficult to assess these things on a continuous basis."

Asked whether the child enjoyed playing with toys, he said: "I haven't witnessed purposeful movements … I think that for much of his waking time, his life is stressful." The hearing continues.

What is CMS?

Congenital myasthenic syndrome (CMS) is an inherited muscle condition that severely limits movement and the ability to breathe. In the UK an estimated 300 people have CMS, with varying degrees of severity. Symptoms usually appear at birth or in early childhood, and include muscle weakness, especially in the face; people are unable to smile and have drooping eyelids. As there is no cure, treatment is aimed at maximising muscle function with drugs and physical therapy. Severe forms of the disease can weaken respiratory muscles and reduce lifespan.